Applications Afloat and Ashore

Figure 2-8.-Some other tackles. Many combinations of single-, double-, and triple- sheave blocks are possible. Two of these combinations are shown in figure 2-8. You can secure the dead end of the fall to the movable block. The advantage is increased by 1. Notice that this is done in figure 2-7. That is a good point to remember. Remember, also, that the strength of your fall—rope—is a limiting factor in any tackle. Be sure your fall will carry the load. There is no point in rigging a 6-fold purchase that carries a 5-ton load with two triple blocks on a 3-inch manila rope attached to a winch. The winch could take it, but the rope couldn’t. Now for a review of the points you have learned about blocks, and then to some practical applications aboard ship: With a single fixed block the only advantage is the change of direction of the pull. The mechanical advantage is still 1. A single movable block gives a mechanical advantage of 2. Figure 2-9.-A yard and stay tackle. Many combinations of single, double, and triple blocks can be rigged to give greater advantages. Remember that the number of parts of the fall going to and from the movable block tells you the approximate mechanical advantage of the tackle. If you fix the dead end of the fall to the movable block you increase the mechanical advantage by one 1. APPLICATIONS AFLOAT AND ASHORE We use blocks and tackle for lifting and moving jobs afloat and ashore. The five or six basic combinations are used over and over in many situations. Cargo is loaded aboard, and depth charges are stored in their racks. You lower lifeboats over the side with this machine. We can swing heavy machinery, guns, and gun mounts into position with blocks and tackle. In a thousand situations, sailors find this machine useful and efficient. We use yard and stay tackles aboard ship to pick up a load from the hold and swing it onto the deck. We use yard and stay tackles to shift any load a short distance. Figure 2-9 shows you how to pick a load by the yard tackle. The stay tackle is left slack. After raising the load to the height necessary to clear obstructions, you take up on the stay tackle and ease off on the yard fall. A glance at the rig tells you that the mechanical advantage of each of these tackles is only 2. You may think it’s hard work to rig a yard and stay tackle when the small advantage is to move a 400-pound crate along the deck. However, a few minutes spent in rigging may save many unpleasant hours with a sprained back. If you want a high mechanical advantage, a luff upon luff is a good rig for you. You can raise heavy loads with this setup. Figure 2-10 shows you what a luff upon 2-4